1. Field of the Invention
The present invention relates to a thin film transistor, a semiconductor device having the thin film transistor, a display device having the thin film transistor, and a manufacturing method thereof.
2. Description of the Related Art
Thin film transistors (hereinafter also referred to as “TFTs”) are widely used already in a technical field of liquid crystal displays. A TFT is a kind of a field effect transistor, and is named after the fact that a semiconductor film for forming a channel formation region is formed with a small thickness. At present, a technique to manufacture a TFT using amorphous silicon or polycrystalline silicon for a thin semiconductor film has already gone into actual use.
A semiconductor material called “microcrystalline silicon” has been familiar for a long time together with amorphous silicon and polycrystalline silicon, and there has been a report on a field effect transistor having microcrystalline silicon (for example, refer to Reference 1: U.S. Pat. No. 5,591,987). Even now, however, TFTs having microcrystalline silicon lag behind amorphous silicon transistors and polycrystalline silicon transistors in actual use, and some reports are made merely at the level of academic conference (for example, refer to Reference 2: Toshiaki Arai et al., “SID 07 DIGEST” 2007, pp. 1370-1373).
A microcrystalline silicon film can be formed over a glass substrate or the like by decomposing a source gas with plasma (weakly-ionized plasma) by a plasma CVD method; however, people think it difficult to control crystal nucleation and crystal growth because reaction proceeds in a nonequilibrium state.
Various researches have been made on microcrystalline silicon. According to a hypothesis, growth mechanism of microcrystalline silicon is as follows: first, a portion of an amorphous phase, in which atoms are distributed randomly, grows over a substrate, and then nuclei of crystals start to grow (refer to Reference 3: Hiroyuki Fujiwara et al., “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)” vol. 41, 2002, pp. 2821-2828). In Reference 3, they think that the density of microcrystalline silicon nuclei can be controlled with the concentration of a hydrogen gas in forming a film because peculiar silicon-hydrogen bonds are observed on a surface of an amorphous portion when nuclei of microcrystalline silicon start to grow.